> Ok, there's a couple of fairly simple calculations that will tell
> you what you need to know, but Microchip is being a _wee_ bit
> optimistic.
>
> For maximum brightness, to minimize I/O load on the PIC, and to get
> the best efficiency from the driver transistors (discreet or in the
> PIC), you should use the longest character on time you can.  All
> LED's should be sequenced in less than 1/30 second to avoid flicker
> (a few rare people can detect flicker at this high rate so you might
> want to use 1/40th or 1/50th sec. instead).  Now divide that value
> by the number of LED's and you have your scan time per character.
> For instance, if you have eight 7 segment LED's, and you want a full
> scan in 1/30th of a second, each character should get 1/240th of a
> second.  At this speed, given a reasonably normal clock speed, you
> will have oodles of time to process other stuff between display
> servicing calls.
>
> Note that because you have to write the character PIC port
> separately from the segment port, you may get some segment "bleed"
> due to the short time that the segment outputs from the PIC are for
> a different character than the character output from the PIC.  The
> simple solution for this is to write a 0 (or FF, depending on CA or
> CC LED's) to the segment port to blank the display just before you
> write to the character port.  Then write the character port,
> followed by the new value for the segment port.  Voila, no more
> bleed.  This applies to other multiplexed display types too, such as
> VFD's.
>
> As for current, there are two parts to this, one for LED current,
> and one for PIC drive current.
>
> In general, to get a multiplexed LED to display the same brightness
> as a direct driven LED you should overdrive each segment by the duty
> cycle of the scan.  That is, in our 8 LED example, if the LED
> display the brightness you need at 20mA per segment when driven
> directly, then you need to drive it at 160mA per segment to get the
> same brightness when multiplexing, since each LED is only on for
> 1/8th of the time.  In my experience, LED's seem to be able to take
> this low duty cycle overdrive with no problems, but if you are
> squeamish about it, you can edge the calculated current down a bit,
> but be aware you may not get the brightness you need.
>
> The other side to the current equation is the PIC drive
> capabilities, and this is where there are a couple of gotchas.  Each
> PIC port pin can source 20mA and sink 25mA, so you can see from this
> that using our calculated drive current of 160mA per segment is
> going to be optimistic at best.  I have no idea whether the maximum
> values specified by Microchip are affected by the duty cycle,
> probably a bit, but even at low duty cycles, sinking 160mA into a
> PIC port may be pushing it.  This of course doesn't even account for
> the character driver port, which must be able to handle the segment
> current times the # of segments.  In our 7 segment example above,
> each character could draw up to 1120 mA, albeit at the 1 in 8 duty
> cycle.
>
> So you can see why Microchip specify a maximum of 7mA per segment in
> the direct drive multiplex example application notes.  You have to
> use some pretty small and efficient LED displays to get adequate
> brightness at 7 mA per segment, especially at low duty cycles.
>
> The other big gotcha, that is not obvious in the PIC specs is that
> although each port pin can source 25mA, the total for _all_ pins is
> 50mA, which really limits the real world drive capability of a PIC,
> IMHO.  That 7mA per segment restriction in the app notes takes this
> into account (7mA time 7 segments = 49mA).
>
> So you can see why, IMNSHO, despite Microchip's examples, it is not
> really practical to drive a multiplexed LED display directly from a
> PIC, at least not the character drive anyway.  You can probably get
> away with driving the segments directly, and driving the characters
> via transistor or other buffers, but this will require fairly
> efficient or small LED's.

> Hi guys,
>
> I'm in need of some advice regarding multiplexing seven seg
> displays. I have 4 displays on 12 i/o pins, and have two
> concerns:
>
> 1) Visibility.
>         What is the optimum "on time" vs "off time" for best
> visibility, while at the same time having enough remaning cpu
> time to be able to handle various other tasks?
>
> 2) Current consumption.
>         How do I determine how much current the leds are going to
> need, and how can I keep these values w/in the current specs of
> the pic?  All the microchip literature seems to suggest that you
> can drive leds directly. I've been doing it that way but have a
> feeling I've been exceeding current specs on the i/o ports.
>
>
> Thanks for any help! :)
>
> -Shane.
>

> On Mon, 15 Sep 1997, Shane Nelson wrote:
>
> > Hi guys,
> >
> > I'm in need of some advice regarding multiplexing seven seg
> > displays. I have 4 displays on 12 i/o pins, and have two
> > concerns:
> >
> > 1) Visibility.
> >         What is the optimum "on time" vs "off time" for best
> > visibility, while at the same time having enough remaning cpu
> > time to be able to handle various other tasks?
> >
> > 2) Current consumption.
> >         How do I determine how much current the leds are going to
> > need, and how can I keep these values w/in the current specs of
> > the pic?  All the microchip literature seems to suggest that you
> > can drive leds directly. I've been doing it that way but have a
> > feeling I've been exceeding current specs on the i/o ports.
> >
> >
> > Thanks for any help! :)
> >
> > -Shane.

> 2mA is indeed very low current for decent brightness on a LED display.  I
> assume by "Packard" you mean this is a Hewlett Packard device?   What is
> the display height, colour, etc.
>
> Something with these specs might just work in Microchip's examples.
>
>
> Martin R. Green
> RemoveMEelimarTakeThisOuTspambigfoot.com
>
> From:  Josef Hanzal[SMTP:EraseMEeuroclassspamspamBeGonePHA.PVTNET.CZ]
>
> Using low current display is really essential. I got good results with
> Packard's HDSP-H111 or HDSP-H101 in triplexed display. Their nominal
> current
> is 2 mA, but I used 1k resistors in series with cathodes which yields about
> 1 mA per segment. At that current the display still shines very nice.
>
> Regards
>
> Josef
>

> 2mA is indeed very low current for decent brightness on a LED display.  I
> assume by "Packard" you mean this is a Hewlett Packard device?   What is
> the display height, colour, etc.
>
> Something with these specs might just work in Microchip's examples.
>
>
> Martin R. Green
> KILLspamelimarspamBeGonebigfoot.com
>
> From:  Josef Hanzal[SMTP:EraseMEeuroclassEraseMEPHA.PVTNET.CZ]
>
> Using low current display is really essential. I got good results with
> Packard's HDSP-H111 or HDSP-H101 in triplexed display. Their nominal
> current
> is 2 mA, but I used 1k resistors in series with cathodes which yields